1. Field of the Invention
The present invention relates to a method for obtaining field strength information and to a circuit arrangement.
2. Description of the Background Art
A method of this type is known from European Pat. No. EP 1 318 623 B1, which corresponds to U.S. Pat. No. 6,922,553, and which is incorporated herein by reference. Inter alia, a distance between the transmitter and receiver can be determined by means of the field strength information and used for detecting relaying. In general, a so-called “RSSI (Received Signal Strength Indicator)” value, which is proportional to the input signal of a receiving antenna, is determined from the received signal strength. In this regard, an input voltage for an operational amplifier is generated from a received electromagnetic signal by means of a receiver unit with an input resistance, and an output voltage is provided by the operational amplifier by means of a fixed amplification factor. The input voltage at the operational amplifier is changed until the output voltage lies within a predefined interval that includes the value of the reference voltage. To this end, the input voltage at the divider node of a voltage divider is tapped and, to adjust the output voltage to the reference voltage, the resistance of the voltage divider is changed by means of connection or disconnection of resistance branches connected to one another in a complex parallel circuit. Furthermore, the method is used to determine the distance of the receiver unit from a transmitting unit from the determined field strength values. Because of the fixedly predefined values of the resistance branches, the spatial resolution is very different depending on the distance; the spatial resolution is insufficient particularly at a short distance.
Inter alia, methods for obtaining field strength information are used in systems for contactless data transmission. An important field of application is identification systems, which are used, for example, for access control in a motor vehicle. Such identification systems consist of a base unit and one or more transponders; such methods are used for determining the field strength values or the distance preferably in transponders, which are designed, for example, as passive and as a result must draw the energy needed for operation by absorption from the electromagnetic field of the base station.